This article was co-authored by Bess Ruff. Bess Ruff is a PhD student of Geography in Florida. She received her MA in Environmental Science and Management from Bren School of Environmental Science & Management, UC Santa Barbara in 2016.

There are 16 references cited in this article, which can be found at the bottom of the page.

A barometer measures air pressure and can forecast the weather within a 12 to 24 hour period. Air pressure can be measured in inches of mercury, millimeters of mercury, or hectopascals, depending on where you live and the gauge on the instrument.[1] In order to tell whether air pressure is rising or falling, you must calibrate the barometer correctly. When you purchase a barometer, it must be adjusted before you can use it to accurately measure atmospheric pressure.

Steps

Part 1

Setting the Barometer

1

Buy a barometer. There are 3 types of barometers that can be purchased. If you own an antique barometer, then is most likely mercury or aneroid. Aneroid or electronic barometers are more commonly available for purchase. Before purchasing a barometer, check its altitude usage. Not all barometers function properly at high altitudes so buy one that can specifically be used at altitude if you live high above sea level. Here are some brief descriptions of each type of barometer:[2]

Mercury: The mercury barometer, sometimes called a stick barometer, was the first to be invented. It uses an open tube system with a pool of mercury that rises and falls with pressure changes. They only work at an altitude up to 1,000 feet.[3]

Aneroid: The aneroid barometer doesn’t use any liquids. It uses a small box made from beryllium and copper that expands or contracts based on pressure changes. These motions cause mechanical hands to move pointing towards the current air pressure.

Electronic: Electronic barometers are a little more complicated to understand, but they employ sensors and strain gauges that cause changes in voltage that can be converted to a display of pressure for the user.

2

Obtain a local reading of barometric pressure. If using an aneroid barometer, you will need to calibrate it to your location. Listen to a local weather forecast to find the current barometer pressure for your location. Make sure the reading is correct for your location. Even a few miles can affect a barometer reading.

Setting your barometer to your location will take into account differences in pressure caused by the altitude of your location.

The factory setting for aneroid barometers is at sea level, but if you don’t live at sea level, you will need to calibrate it.[4]

3

Set the indicator hand on your barometer. Locate the small adjusting screw on the back of your barometer. With a small screwdriver, turn the adjusting screw to move the hand to your location's current pressure. Watch the face of the dial and stop turning the screwdriver when the hand reaches the appropriate reading.

If you are using a mercury barometer, you will have to use a conversion for your readings.

Digital barometers have sensors to calibrate altitude automatically.

4

Hang the barometer in a location that works for you. It makes no difference whether the barometer is hung on an inside or an outside wall. The pressure will be the same both indoors and outdoors.[5]

Well-sealed and air-conditioned rooms are not as affected by changes in air pressure, so avoid these rooms if possible.

Avoid a location that is exposed to direct sunlight as the temperature changes can affect the readings.

Hang the barometer away from drafty locations, like near a door or a window. Air pressure is too variable in these locations.

5

Check occasionally to be sure that your barometer works properly. If you suspect inaccurate readings, you can check your barometer with an easy trick. With the barometer hanging on the wall, slowly slide the bottom of it sideways to a 45-degree angle.[6]

If you have a mercury barometer, the mercury should rise to the top of the tube and produce an audible "tick" sound that you may also feel. The tube should fill with mercury.

If you have an aneroid barometer, the indicator hand should rotate clockwise around the dial.

If your barometer fails this test, you may need to have it professionally serviced and re-set before you can count on its accuracy. However, most barometers can last years with no servicing.

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Part 1 Quiz

Where should you hang your barometer to ensure it gives you accurate readings?

A well-sealed room

Not exactly! A room that’s too tightly-sealed isn’t prone to be affected by changes in air pressure. Because of that, the barometer won’t give you a good reading of the air pressure in the general area. Pick another answer!

A wall that gets sunlight exposure

Nope! Anywhere that gets direct sunlight is a bad place to hang a barometer. Anything that significantly affects the barometer’s temperature can skew the readings. Choose another answer!

Near a window

Definitely not! Anywhere that has a highly variable air pressure is a bad place to hang a barometer. The instrument won’t be able to get a stable reading if windflow is too dynamic. Pick another answer!

Away from drafts

Yup! Hang your barometer anywhere that isn’t drafty. That means avoid setting it near doors or windows, as the variable air pressure there can skew the readings. Also avoid putting the barometer in sunlight or anywhere temperature-controlled. Read on for another quiz question.

Part 2

Using the Barometer

1

Set the manual hand to the current reading. Turn the barometer's center knob so that the arrow rests directly above the indicator arrow (this is the current barometric pressure for your location). You can identify the set hand by its notched arrow about halfway down the hand.[7]

The set hand will serve as a reference that allows you to easily tell if the pressure is steady, rising or falling.

Remember, this hand will only be present on an aneroid barometer. If you have an electronic barometer, you can simply check the reading.

If you have a mercury barometer, you will need to correct for altitude if you’re above sea level.

2

Correct for altitude if you’re using a mercury barometer. To accurately measure the air pressure using a mercury barometer, you will need to correct for your altitude using a conversion chart, which can be found online.[8] Look at the barometer at eye level and record the number next to the top of the mercury. This is the pressure in millimeters of mercury (mmHg).

Find your elevation and then use the chart to find the relevant correction factor. Add the correction factor to the reading on the barometer. This reading should match the reading of the local weather service.

If you are at an elevation of over 1,000 ft, mercury barometers do not work.[9]

3

Check the barometer an hour later. Predicting weather using a barometer is all about changes in air pressure. You want to check the reading every few hours to determine if the pressure is changing or staying the same.

If using an aneroid or mercury barometer, gently tap the face of the barometer to release any pressure changes stored in the mechanisms. Take the reading after the needle or mercury has stopped moving.

For the aneroid barometer, move the set hand if the pressure has changed so the next time you check it will be obvious what direction the air pressure is going.

4

Chart the changes in pressure. Keep a journal of all the readings you take with your barometer. Sketch a small graph for the changes in a day to help with your forecasting. Is the pressure rising? Falling? Staying the same? This is all important information for predicting the weather.[10]

Do not expect large changes in the movement of the needle. Daily changes are usually between 0.02 and 0.10 of an inch using the barometer scale. Variations are dependent upon location and altitude.[11]

Take frequent readings (every few hours) and plot them on your graph.

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Part 2 Quiz

At what elevation will a mercury barometer stop working?

100 ft

Not exactly! Mercury barometers function well at 100 feet. It’s when you’re working with altitudes a bit higher that you run into problems and may need to switch over to an aneroid barometer! Try again...

500 ft

Nope! Even at this high an altitude a mercury barometer should give you accurate readings. You don’t need to rely on an aneroid barometer instead if you’re at just 500 feet. Try again...

1000 ft

Right! At 1000 feet, a mercury barometer will not function and will not give accurate air pressure readings. That’s okay, though, because there are alternatives. Instead, use an aneroid barometer, which will work at higher elevations! Read on for another quiz question.

Mercury barometers work at any elevation

Not necessarily! Mercury barometers can work at fairly high elevations, but they do have a breaking point. Past a certain altitude, mercury barometers do not function. There’s a better option out there!

Part 3

Forecasting the Weather

1

Predict rain if the air pressure is falling. Generally, if the pressure is falling, the weather is taking a turn towards storms and rain. The starting point of the reading is also important in the forecast. Higher readings indicate better weather even if the pressure is falling.[12]

If the reading is over 30.2 inches of mercury and falling rapidly, this indicates cloudy, but warmer weather.

If the reading is between 29.8 and 30.2 inches of mercury and falling rapidly, rain is most likely on the way.

If it’s under 29.8 inches of mercury and falling slowly, rain is likely; if it’s falling rapidly, a storm is imminent.

2

Forecast improving weather when air pressure rises. As the air pressure rises, the weather tends to improve as the high pressure systems moves through your location.[13]

Readings over 30.2 inches of mercury that rise indicate that the weather will continue to be fair.[14]

Readings between 29.8 and 30.2 inches of mercury that rise indicate that the weather will remain whatever it presently is.

Readings under 29.8 inches of mercury that rise indicate that the weather is clearing, but will be cooler.

3

Forecast fine weather when air pressure is steady. Steady air pressure indicates long periods of nice weather and suggests that you will be experiencing more of the same. If it’s sunny and the pressure is holding, expect more sunshine! Higher pressures indicate warmer weather, while lower pressures indicate cooler weather.[15]

A strong high pressure system is around 30.4 inches of mercury. Anything above 30 is considered high pressure.[16]

A typical low pressure system is around 29.5 inches of mercury. Anything below 29.9 is considered low pressure.

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Part 3 Quiz

What kind of weather does an air pressure reading of 30.4 indicate if the air pressure is falling rapidly?

Cloudy

Yes! If a barometer reads out a mercury level of 30.4 when the air pressure is rapidly falling, this indicates that it’ll soon get cloudy. However, it also means warmer weather is on the way. Don’t get the ponchos and umbrellas out just yet! Read on for another quiz question.

Rainy

Nope! A 30.4 mercury reading in these conditions doesn’t indicate rainy weather. You should be prepared for rain if the reading is anywhere between 29.8 and 30.2 inches of mercury and falling quickly. Choose another answer!

Fair

Not exactly! A 30.4 mercury reading when air pressure is falling rapidly doesn’t quite indicate fair weather. If air pressure was rising, however, this reading would be an indication of continued fair weather on the way. There’s a better option out there!

Stormy

Try again! This mercury reading doesn’t indicate a storm on the way. You should be wary of a storm when the mercury reading is below 29.8 and falling rapidly. There’s a better option out there!

Community Q&A

Set the indicator hand on your barometer. Locate the small adjusting screw on the back of your barometer. With a small screwdriver, turn the adjusting screw to move the hand to your location's current pressure. Watch the face of the dial and stop turning the screwdriver when the hand reaches the appropriate reading.

Any dial barometer aneroid or mercury will always have some amount of 'sticktion' therefore a GENTLE tap is normal. (Tapping actually helps the user as it indicates which way the barometer is moving at the time it is being looked at.).

I've adjusted the manual hand to correct pressure by turning the screw at the back - what reading should the other hand (turned by the central knob) be set to?

wikiHow Contributor

Community Answer

The golden hand is an indicator. Once you have adjusted the correct pressure reading with the black hand, you adjust the black hand over on top of the golden hand. When the pressure changes, the golden hand is used to determine the changes in atmospheric pressure.

To set a barometer, start by listening to a local weather forecast or going online to find a reading of your local barometric pressure. Then, insert a screwdriver into the adjusting screw on the back of your barometer and turn it to move the hand on the front of the device. Keep turning the screwdriver until the hand is lined up with the barometric pressure for your area. When you're finished, remove the screw and hang up your barometer. To learn how to read a barometer, scroll down!

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Article Info

This article was co-authored by Bess Ruff. Bess Ruff is a PhD student of Geography in Florida. She received her MA in Environmental Science and Management from Bren School of Environmental Science & Management, UC Santa Barbara in 2016.

"I have always been interested in barometers so I bought my late father one years ago, which I inherited when he passed. Prior to reading this article, I did not understand the value of noting the position of the black arm which enables more detailed and accurate reading of the changes. I now understand that the altitude of where you live is also relevant and barometers need servicing. Thank you so much."..." more

Rated this article:

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Rob Y.

Jan 20, 2018

"I have an old, at least 70 years or more, J.G.G. castle with 3 turrets symbol on the back of my barometer. It has a dial in inches/mercury and a thermometer above it. Been in my family since I was a child. I reset it when I moved here 40 years ago and it's never missed a beat. Gives exact same readings as my digital weather station I got a few years back, and doesn't need batteries! Doesn't give me sun/moon rise/set times though, but I've got eyes for that! I love it. "..." more

AB

Annette Barnes

Aug 3, 2016

"The other day I found this old Barometer we had, I cleaned it up and turned out so beautiful, I had to know how to use it. Got on this site, got info on how to use the barometer and found out things I never knew about our weather. Awesome, it helped me a lot."..." more

RK

R. K.

May 5, 2016

"Easy to read and understand. Step-by-step pictorials are very helpful (humanitarian help), in particular for readers with diminishing eyesight. No need to recite a highly complicated explanatory description of barometer adjusting here. Kudos!"..." more

KG

Karen Guthrie

Jul 6, 2016

"I bought a barometer for my boyfriend as a birthday gift. It did not come with instructions on setting it, so I came here from a Google search. Your article was very straightforward and easy to understand. Thank you for the help!"..." more

RG

Rody George

Jul 28, 2016

"I have learned a lot about my inherited barometer: that it is aneroid, how to use it, how to understand my "fatigues" sometimes. Thank you."..." more

FR

Frank Reid

Nov 10, 2017

"Since retiring, I have found that tinkering with science and electronics is more fun than watching the paint dry."

Rated this article:

AS

Al Simmons

May 21, 2017

"It helped me understand how the barometer works, how to use it and adjust it correctly."

TC

Tina Couper

Nov 4, 2017

"Yes, this helped. I looked this up for my mother and now would like one of my own."